In the lithography process for manufacturing semiconductor integrated circuit devices, a pattern transfer by an optical reduction projection exposure with a photomask (sometimes, called reticle) is generally conducted. Electromagnetic waves used for the exposure are i-line (having a wavelength of 365 nm), KrF excimer laser light (having a wavelength of 248 nm), ArF excimer laser light (having a wavelength of 193 nm), or F2 excimer laser light (having a wavelength of 157 nm), of which wavelengths are shortened for improving resolution. Since there are materials having very high transparency to such electromagnetic waves such as quartz (including fluorine-doped quartz), the structure of the photomask is ordinarily composed of a quartz substrate and a patterned absorber. That is, a photomask blank as a starting form for making a photomask is generally prepared by depositing an absorber on one surface of a quartz substrate. In this case, since the absorption coefficient of the quartz at the exposure beam as mentioned above is very small (i.e., the transparency is very high), there is little limitation on thickness of the substrate. Therefore, the shape of the photomask blank is defined in consideration for securing of required pattern region and rigidity of the mask. Currently, the shapes of photomask blanks are standardized, including a rectangular parallelepiped 6 inches, 7 inches, or 9 inches on a side, by SEMI (Non-patent document 1). The shape of the latest photomasks is now a square 6 inches in edge length and on a side and 0.25 inches in thickness.
On the other hand, as a pattern transfer method used in the lithography process for manufacturing semiconductor integrated circuit devices, there is a method in which a lithography mask (hereinafter, referred to simply as “mask”) is irradiated with X-ray, extreme ultraviolet beam, or the charged-particle beams and a resist is exposed to the X-ray, the extreme ultraviolet beam, or the charged-particle beams which penetrate the mask with intensities determined by the mask pattern. As used herein, the X-ray means an electromagnetic wave having a wavelength on the order of 0.5 nm, the extreme ultraviolet beam means an electromagnetic wave having a wavelength of 13 nm, and the charged-particle beams mean electron beams or ion beams.
There is no material having high transparency relative to the X-ray, the extreme ultraviolet beam, or the charged-particle beams, just like quartz for electromagnetic waves used in photolithography. Therefore, the mask can have the so-called stencil membrane structure or continuous membrane structure for the X-ray, the extreme ultraviolet beam, or the charged-particle beams. The stencil membrane structure is a structure in which patterned through-holes “h” are formed in a self-supporting membrane “m” (hereinafter, referred to as “membrane”) as shown in FIGS. 5(a) and 5(b). On the other hand, the continuous membrane structure is a structure in which a patterned absorber or scatterer “a” is formed on a membrane “m” as shown in FIG. 5(c).
As projection lithographies using electrons having high energy of 100 keV, there are the PREVAIL method (Non patent document 2) and SCALPEL method (Non patent document 3). The mask may have either of the stencil membrane structure and the continuous membrane structure. In the case of the continuous membrane structure, the mask has a patterned scatterer formed on a membrane. As for electron lithography masks for the PREVAIL method, a stencil membrane structure consisting of a silicon layer having a thickness of 2 μm has been reported. As for electron lithography masks for the SCALPEL method, a continuous membrane structure employing a 150-nm-thick silicon nitride (SiNx) layer as a membrane, and a complex layer comprising a 6-nm-thick chrome (Cr) layer and a 27-nm-thick tungsten (W) layer as a scatterer has been reported. In any of the electron projection lithographies, the mask is arranged to face a wafer coated with a resist on each side of a projection electron optical system and the transfer ratio is ¼.
In a low-energy electron-beam proximity projection lithography (Non patent document 4) using electrons of low energy of from 2 to 5 keV, a mask to be used has the stencil mask structure. As such a membrane, a silicon carbide (SiC) or silicon (Si) layer of from 0.3 to 0.5 μm in thickness has been reported. The mask is placed in proximity to a wafer with a resist applied thereon, facing the wafer. The distance between the mask and the wafer is from 50 to 40 μm.
In the lithography using extreme ultraviolet beam, a mask to be used generally is of reflective type, but a mask of transmissive type has been also reported. In this case, the mask structure may be either the stencil membrane structure or the continuous membrane structure.
In the proximity X-ray lithography, a mask to be used has the continuous membrane structure. For example, a mask employing a 2-μm-thick silicon nitride (SiNx) layer as the membrane and a 0.5-μm-thick tantalum (Ta) layer as the absorber has been reported. The mask is placed in proximity to a wafer with a resist applied thereon, facing the wafer. The distance between the mask and the wafer is from 20 to 5 μm.
In the proximity X-ray lithography and the low-energy electron-beam proximity projection lithography, the transfer ratio is 1. Therefore, one or several semiconductor integrated circuits to be transferred are disposed in a pattern region. If a large number of semiconductor integrated circuits are disposed, the number of semiconductor integrated circuits to be transferred at once is increased so as to increase the productivity, while a problem of deteriorating the positional accuracy of pattern is caused in case of a mask having a large area membrane. Therefore, the size of the pattern region is limited to a square 20-50 mm on a side. In the transfer at the wafer, a method of moving the transferring region successively in a step-and-repeat or step-and-scan mode is employed.
In terms of the mask structure, the mask for the proximity X-ray lithography (hereinafter, referred to as “proximity X-ray mask”) and the mask for the low-energy electron-beam proximity projection lithography (hereinafter, referred to as “proximity electron-beam mask”) have similar structures. Therefore, in the following description, these are collectively handled. When you heed that the absorber in the proximity X-ray mask and the scatterer of the proximity electron-beam mask correspond to each other, it is apparent that one structure can be easily employed as the other structure, except the pattern region.
Hereinafter, the structure of the conventional lithography mask will be shown in FIGS. 5(a)-5(c), FIGS. 6(a) and 6(b), and FIGS. 7(a) and 7(b), focusing on its shape. FIGS. 5(a) and 5(b) are a plan view (a) and a sectional view (b) of a proximity electron-beam mask, respectively. The shape of a mask 10 itself has the form of a silicon wafer because a silicon wafer is employed as a substrate 11. The substrate 11 is provided at its center 12 with a pattern region of square about 50 mm on a side over an opening 13 and is further provided with a perforated pattern 14 formed in the pattern region. The substrate 11 has a wafer shape of 4 inches to 8 inches. Actually, a proximity electron-beam mask with a substrate 11 having an outer diameter of 4 inches is manufactured. In the above description, the discussion whether the material of the perforated pattern 14 and the material of the substrate are the same or not is omitted because the above description is made only focusing on the shape.
FIG. 5(c) is a sectional view of a proximity X-ray mask 20 without a plan view. A substrate 21 has a wafer shape. The substrate 21 is provided at its center 22 with a pattern region over an opening 23 and is further provided with an absorber or scatterer pattern 24 fixed to the pattern region.
Shown in FIGS. 6(a) and 6(b) is a proximity X-ray mask 30. A substrate 31 has a wafer shape. Similarly to FIG. 5(c), the substrate 31 is provided at its center 32 with a pattern region over an opening 33 and is further provided with an absorber or scatterer pattern 34 fixed to the pattern region. The substrate 31 is fixed onto a frame 35 of which the outermost shape is a wafer shape similar to the substrate and which is provided with an opening 36 corresponding to the opening 33. According to the NIST standard, a mask comprising the substrate 31 having an outer diameter of 4 inches and the frame 35 having an outer diameter of 5 inches are established. A product example is shown in Non-patent document 5. A product example of a proximity electron-beam mask having a similar structure is reported in Non-patent document 6.
Shown in FIGS. 7(a) and 7(b) is another proximity X-ray mask 40. A substrate 41 has a wafer shape. The substrate 41 is provided at its center 42 with a pattern region over an opening 43 and is further provided with an absorber or scatterer pattern 44 fixed to the pattern region. The substrate 41 is fixed to a frame 45 which has a square shape and is provided with an opening 46 corresponding to the opening 43. Similarly, a low-energy electron-beam mask comprising a substrate having an outer diameter of 4 inches and a thickness of 0.525 mm, and a frame of a square about 6 inches on a side and a thickness of 5.82 mm (that is, the total thickness of the frame and the substrate is 0.25 inches) has been proposed.
Hereinafter, the term “substrate” is used to express a substrate itself (e.g., 31 in FIG. 6) or a substrate plus an absorber or scatterer (e.g., 31 plus 43 in FIG. 6) because the presence of the absorber or scatterer would not affect this invention. Thus it can be stated that the mask in FIG. 6 comprises the substrate and the frame.
It is apparent from the above examples that, as for the pattern region, a 4-inch wafer is satisfactorily employed as the mask substrate. In either the conventional example 2 shown in FIGS. 6(a) and 6(b) or the conventional example 3 shown in FIGS. 7(a), 7(b), the frame 35, 45 has a function of increasing the rigidity of the entire mask by fixing the mask substrate 31, 41 to the frame 35, 45 so as to prevent the mask substrate from deforming during manufacturing of the mask, thereby ensuring the positional accuracy of the pattern and facilitating the handling of the mask during transportation and in a transferring apparatus.
On the other hand, in the conventional example 1 shown in FIGS. 5(a)-5(c), a wafer of 8 inch size is employed as the substrate 11, 21. This is because, by employing a large wafer as the substrate 11, 21, a portion of the substrate other than the pattern region is adopted to have the same function as the frame has. Further, since the electron lithography mask for the PREVAIL has a wafer shape of 8 inch size, the same shape is employed for the proximity electron-beam mask and the electron lithography mask. Therefore, there is an advantage of promoting the common use of the manufacturing apparatus.
The relation between a proximity electron-beam mask or a proximity X-ray mask as described in the above and a mask blank (hereinafter, referred to as just “blank”) before the mask is manufactured will be described. Normally, masks in commercial mass production are processed, starting with blanks as products in intermediate stage but not with substrates. A blank is subjected to resist patterning (for example, resist coating, electron beam writing, resist development/rinsing) and etching so as to form a structure of a perforated pattern (see FIG. 5(b)) or an absorber or scatterer pattern (see FIG. 5(c)). The scope of the present invention as will be described later also covers cases in which a similar mask structure is produced even when manufacturing processes are out of the aforementioned sequence, for example, the process does not start with the blank. Therefore, the structure of a mask is defined by the structure of a blank. The structure of blanks will be discussed below.
[Patent document 1]
Japanese Patent Unexamined Publication 2002-299229
[Patent document 2]
Japanese Patent Unexamined Publication H08-306614
[Non-patent document 1]
SEMI P1-1101: Specification for Hard Surface Photomask Substrates
[Non-patent document 2]
H. C. Pfeiffer, Journal of Vacuum Science and Technology B17 p. 2840 (1999)
[Non-patent document 3]
L. R. Herriott, Journal of Vacuum Science and Technology B15 p. 2130 (1997)
[Non-patent document 4]
T. Utsumi, Journal of Vacuum science and Technology B17 p. 2897 (1999)
[Non-patent document 5]
Y. Tanaka et al., Proceeding of SPIE 4409 p. 664 (2001)
[Non-patent document 6]
K. Kurihara et al., Proceeding of SPIE 4409 p. 727 (2001)
The conventional example 1 shown in FIGS. 5(a)-5(c) and the conventional example 2 shown in FIGS. 6(a)-6(b) have a disadvantage that a photomask manufacturing apparatus is hardly employed in processes for manufacturing a mask, such as an electron beam writing process, a dry-etching process, a particle-detecting process, a defect-detecting process, and a pattern-size-measuring process by a scanning electron microscope. If the photomask manufacturing apparatus can be employed, it confers tremendous technical benefits and economical effects. A great benefit is received particularly in the electron beam writing process.
Since the pattern on the photomask is written by an electron beam writer, the writer for photomasks is specialized for photomasks so that the writer for photomasks can not write high-accuracy pattern to a substrate not having a photomask shape.
There are two types of systems for fixing a blank in the writing chamber of an electron beam writer, one of which is a cassette system and the other of which is a cassette-less system.
The cassette system is a system in which a photomask blank or a wafer-shaped blank is set to a special carrier called a cassette (sometimes called a pallet), the cassette with the blank set to it is entered into the writing chamber, and the writing is conducted in a state that the blank is fixed to the cassette.
On the other hand, the cassette-less system is a system in which a photomask blank is directly entered into the writing chamber and is fixed by a fixing mechanism on the table, that is, a system without using a cassette.
The cassette system has been employed in most of conventional electron beam writers, but now the kind of electron beam writers of the cassette type is decreased and the trend of technology directs to the cassette-less type. The reason is that the cassette-less type provides easier control of the temperature of photomasks and easily provides stable and high positional accuracy of pattern because the deformation of the photomask blank fixed to the fixing mechanism is unambiguous without being affected by the used cassette.
As a blank of a type on which pattern can be written by an electron beam writer designed and made exclusively for photomasks, the structure of the conventional example 3 shown in FIGS. 7(a), 7(b) has been proposed. However, if one attempts to use the electron beam writer for photomasks to the blank of this example, one will find a problem that it is hard to keep the surface to be patterned (hereinafter, referred to as a pattern region surface) in a focusing range of electron beams. This point will now be described in detail.
Hereinafter, an example of a cassette-less-type writer will be explained. FIGS. 8(a) and 8(b) are a plan view (a) and a side view (b) schematically showing a preparation arrangement for writing onto a conventional photomask blank 50. The photomask blank 50 placed at a fixed position on the table within the writing chamber of an electron beam writer for photomasks is arranged as shown in FIGS. 8(a) and 8(b). It should be noted that the upper surface of the photomask blank 50 is covered with an electron beam resist, but not shown.
In this example, the photomask blank 50 is pressed against hemispheroids 52, which are made of ruby or the like and fixed on the lower surfaces of end portions of three fixing arms 51 in the electron beam writer for photomasks, by lifting mechanisms 53 from the bottom. An area defined by the tips (hereinafter, referred to as “reference points A”) of the three hemispheroids 52 is a reference surface for defining the upper surface position of the photomask blank 50 for writing. The positions of the three reference points A depend on the manufacturers of the electron beam writer for photomasks. Normally, the positions are at a distance of from 5 to 10 mm from the outer periphery of the substrate of the photomask blank 50. When the pattern region of the photomask blank 50 is irradiated with electron beams by using the electron beam writer for photomasks, the focusing surface of electron beams is normally adjusted relative to the aforementioned reference surface. If the pattern region surface (the upper surface coated with the electron beam resist) of the photomask blank 50 shifts largely from the electron beam focusing surface, it causes deterioration of a resist pattern image. The allowable shift in practical use depends on the required resolution of subject pattern, but generally from 10 to 30 μm.
In the case of using a cassette, the cassette is provided with a mechanism for fixing a photomask blank 50 and defining a reference surface similarly to that shown in FIGS. 8(a) and 8(b). Therefore, in the cassette-type writer, the focusing surface of electron beams, when the cassette is loaded in the writing chamber, is aligned to the aforementioned reference surface.
As mentioned above, the reference surface in the writing chamber of the electron beam writer for photomasks is set in one plane as the pattern region surface of the photomask blank 50. For more details, the photomask blank 50 supported by three points on the outer periphery may be curved spherically at its center by its own weight. The degree of the deflection of the pattern region surface can be estimated theoretically. Accordingly, the influence on positional accuracy of pattern can be reduced to a negligible degree by correction during writing.
In a height measurement conducted before the writing with this arrangement, as shown in FIG. 8(b), the height of the pattern region surface is measured at several points. FIG. 8(b) shows a system in which two laser beams 541, 542 are incident on and thus reflected by the pattern region surface and the reflected beams are caught by two CCD line sensors (not shown). In one setting example, when the height of the pattern region surface from the reference surface measured by this measurement system exceeds a specified value (for example, 20 μm), the writing is cancelled.
FIG. 9 is a schematic drawing showing a case of writing preparation in which a blank 40′ for a mask 40 of the conventional example 3 shown in FIGS. 7(a) and 7(b) is fixed to the table in the writing chamber of the electron beam writer for photomasks. It should be noted that the upper surface of the blank 40′ is covered with an electron beam resist, but not shown. The reference surface defined by the three reference points A and the pattern region surface of the blank 40′ is away from each other by the thickness d of a substrate 41 (in some cases, including the thickness of an adhesive layer, i.e., normally from 0.4 to 0.6 mm).
Another example of the cassette structure different from the above example in a cassette-type writer will be explained. Instead of the three sets of fixing arms, hemispheroids, and lifting mechanism in the example of FIGS. 8(a) and 8(b), the cassette of this example has three sets on the left side and three sets on the right side, i.e., six sets in total of fixing arms, hemispheroids, and lifting mechanisms. These are positioned at a distance of from 5 to 10 mm from the outer periphery. The substrate of the photomask blank 50 fixed to the cassette may be curved into a cylindrical shape by its own weight. The degree of the deflection of the pattern region surface can be estimated theoretically. Accordingly, the influence on positional accuracy of pattern can be reduced to a negligible degree by correction during writing. It should be noted that the reference surface of this example is not unique and is a curved surface tangent to the tips of the six left and right hemispheroids. The discussion about the aforementioned example is applicable to this example too.
In the normal arrangement for writing on a photomask blank, the focusing surface of electron beams is aligned relative to the reference surface as mentioned above. In the conventional example 3 shown in FIGS. 7(a) and 7(b), writing is conducted after aligning the focusing surface of electron beams relative to the blank 40′ for the mask 40 according to information of the height of the pattern region surface. If the focusing surface of electron beams is apart from the reference surface by 0.3 mm or more, a problem of not obtaining high positional accuracy is caused. In the form that the substrate 41 and the frame 45 are fixed to each other, it is difficult to keep the upper surface of the substrate 41 parallel to the upper surface of the frame 45. Accordingly, there may be a problem that the height of the pattern region surface shifts largely from the focusing area of electron beams due to the inclination of the upper surface of the substrate 41.
In the electron beam writer for photomasks, a positioning mechanism is employed on the presupposition that the shape of a substrate of a photomask blank is rectangular. That is, the photomask blank is positioned such that the outer sides of the photomask blank are parallel to the x-y coordinate axes of the table and is then fixed. However, in the case of writing to blanks for masks 10, 20, 30 having a round shape as shown in FIGS. 5(a) and 6(a), the positioning mechanism equipped in the electron beam writer for photomasks is useless. Consequently, there is a problem that it is not possible to accurately write a mask pattern on the pattern region at the center of the blank.